Permanent Stormwater Management Measures

Chapter 5

Permanent Stormwater Management Measures5.1 Stormwater Avoidance and Minimization Approaches

5.2 Summary of Stormwater Management Techniques and Control Measures

5.3 Management Techniques

5.4 Stormwater Control Measures (SCMs)

5.5 Inlets, Outlets and Flow Control

What’s in this Chapter?Section 5.1 describes approaches to be taken in the concept and preliminary site design layout phasesthat will avoid and minimize the generation of stormwater runoff from new and redevelopment projects.

Section 5.2 provides an overview of the different stormwater control measures and managementtechniques.

Section 5.3 describes management techniques for establishing land use cover and protocol on how toapply specific techniques.

Section 5.4 provides engineering design specification and management protocol for individual stormwatercontrol measures (SCMs) that can be used in stormwater management systems to achieve Smart SiteDesign goals for runoff reduction, pollutant removal, andpeak flow control.

Section 5.5 describes and provides design protocol for elements that are common to most SCMs such aspretreatment, inlets and outlets.

n5.1 Stormwater Avoidance and Minimization Approaches

5.1.1 Protect Sensitive and Special Value Resources

This source control approach seeks to protect the innate characteristics of the landscape that absorbrainfall and provide other ecological services, such as wildlife habitat, runoff filtration, and nutrient cycling.Conservation of natural features and environmental resources help maintain the predevelopmenthydrology of a site by reducing runoff, promoting infiltration, preventing soil erosion, and protecting areasthat function as pockets of rainfall retention. Examples of these features include forests, wetlands, nativegrasslands, floodplains, riparian corridors, zero-order stream channel, springs and seeps, ridge tops, andshoreline buffers. In general, conservation of these areas should maximize contiguous area and avoidfragmentation.

Zero-order streams (or headwaters) provide important watershed functions, including groundwaterrecharge and discharge, nutrient storage and transformation processes, storage and retention of erodedhill slope sediments, and delivery of leaf and woody debris inputs. Compared to high-order streams, zero-order streams are disproportionately disturbed by mass grading, burying, or channelization.

The conservation approach should be implemented during the conception of the project plan. First,preserve the natural/native topography of a site as much as possible, then delineate drainage andconveyance patterns and protect these areas as much as possible, and finally, ensure that adequatebuffers surround these areas including the largest buffer around the sensitive headwaters and wetlandhabitats. Continuing with the project-planning phase from this perspective will ensure that these sensitive

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Tennessee Permanent Stormwater Management and Design Guidance Manual 55

hydrologic areas are preserved and remain linked to other habitat corridors from adjacent areas, minimizethe need for grading, and maximize the land’s natural retention processes.

Preserving natural areas may also provide secondary benefits of reducing noise pollution, providingvaluable wildlife habitat, and creating scenic views and aesthetically-pleasing spaces. These benefits mayequate to increased property value as well as quality of life components within the community.

5.1.2 Minimize Land Disturbance

This source control approach seeks to limit the degree of clearing and grading on a project site in orderto prevent soil compaction, conserve native soil structure, prevent erosion, and protect zero-order streams.This approach is applied by setting grading limits to minimize the total site area that must be cleared andgraded, and/or minimizing the disturbed site area at any one time by completing the project in phases,stabilizing one phase as the next phase is being cleared. This is accomplished by 1) identifying key soils,drainage features, and slopes in the initial site inventory, and then 2) establishing limits of disturbancebeyond which construction equipment is excluded. Minimizing grading limits may reduce overall landscapecosts as well as help preserve the natural character and aesthetic of a site, which may increase propertyvalue.

Specific landscape features on a site may also be more prone to contributing runoff and/or sediment erosion.Avoid disturbing long, steep sloped areas. If these areas are cleared, by essence of gravity and thepropensity of water to concentrate and carry energy, then these areas may contribute more runoff anderosion relative to flatter areas. Furthermore, the footprint of impact for grading on a slope is greaterthan that on a flatter slope because of the efforts required to create a level building pad. General guidelinesfor slope development are listed in the Table 5.1. Development on slopes > 25% is strongly discouraged.

Table 5.1: Guidelines for slope development related to soil erosion risk exposure (Prince George’s County, 2000).

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56 Tennessee Permanent Stormwater Management and Design Guidance Manual

Grade Maximum Slope Length (ft) Erosion Risk

0-7% 300 Low

7-15% 150 Moderate

Over 15% 75 High

5.1.3 Reduce and Disconnect Impervious SurfacesA broad variety of actions can be taken to minimize the creation of new impervious surface and allow fordisconnection of needed/existing impervious surface. Collectively, with the other avoidance andminimization approaches and stormwater control measures, these actions are key elements to the smartsite design. The following is a list of common impervious surface reduction approaches in residential andcommercial settings:

- Reduce residential street width, street right-of-way width, and cul-de-sac radius.- Use swales and other linear control measures that can be located within the right-of-way.- Install bioretention or vegetation on the island in the center of the cul-de-sac or other unused

space.- Use narrow sidewalks on one side of the street only, or move pedestrian pathways away from

the street entirely.- Provide greenspace for downspout disconnection of rooftops from storm drain systems.- Minimize driveway length and width or use shared driveways.

- Allow for cluster or open-space designs (e.g. zero lot line) that reduces lot size or setbacks inexchange for conservation of open spaces.

- Use alternative permeable pavements instead of impervious surfaces. - Design buildings and parking to have multiple levels.- Store rooftop runoff in green roofs, foundation planters, bioretention areas, or cisterns.- Reduce parking lot size by reducing parking demand ratios and stall dimensions.- Use landscaping areas, tree pits, and planter boxes for stormwater runoff treatment.

Existing local development codes and ordinance may discourage or even prohibit the application of theseelements of smart site design. Impervious surface reduction must be applied at the site layout phase ofa project in order to be effective. Check with local jurisdictions to ensure that plans to incorporate theseelements into a project design will not be contrary to any code or ordinance. If so, the local stormwaterprogram should consider making changes to not only allow for these beneficial actions but, ideally,encourage their wide use.

Impervious surface disconnection is a stormwater control strategy that incorporates an infiltration zoneinto a site plan that is engineered to accept runoff from an adjacent rooftop, pavement, or other impervioussurface (See Sec 5.4). While the detailed design of the infiltration zone is completed using a ratio of greenarea to impervious surface area, the disconnection approach needs to be implemented in the conceptphase such that the site layout includes adequate areas for infiltration between impervious surfaces andstormwater drainage infrastructure or natural drainages.

n5.2 Summary of Stormwater Management Techniques and Control Measures

Chapters 1-4 of this manual are a guide to integrating permanent stormwater management systems intodevelopment projects, including the conceptual, preliminary, final, and build-out phases. This approachimproves upon traditional practices used across Tennessee, shifting from “end of pipe” and “singlepurpose solutions” to an integrated design, which includes systems of stormwater managementtechniques and control measures discussed later in this chapter.

The remainder of this chapter provides detailed guidance for the proper design and application ofstructural SCMs as well as management techniques that preserve and restore the intrinsic value andhydrological performance of the land. These landscape-based SCMs require a new approach to sitedesign where landforms, soils, and vegetation are used together with structural SCMs to effectivelyachieve the required runoff reduction, pollutant removal and other site-specific goals. Developers andtheir designers are encouraged to integrate and combine management techniques and SCMs presentedin here to achieve stormwater management goals and optimize preservation and restoration of naturalfeatures that play critical roles in hydrologic processes.

Each SCM is covered in this chapter, including an overview fact sheet and detailed review of function,design, construction requirements, and maintenance and management. Selecting the best SCM for agiven development is heavily dependent on existing site characteristics (slopes, soil type, contributingarea use, etc.) as well as the ultimate intended function, benefits and long-term operations of the SCM.Table 5.2 compares benefits of SCMs relative to each other on a categorical scale of low-moderate-high.Use this information along with additional pertinent considerations when selecting SCMs.

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Land Cost5

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Chapter 5.3

Management TechniquesInstalling SCMs prior to site preparation may result in failure of the measures due to sedimentation and/orcompaction. Timing their installation (pre-planning) is important to protecting these SCMs and avoidingsometimes extensive post-construction rehabilitation or complete re-installation. Infiltration SCMs dependon quality of soils and their porosity. These SCMs depend on soil mixtures, types, and pH. For example, astandard functional bioretention facility should have sand, loam and compost mixture. No other materialsor substance should be mixed in it that may be harmful to plant growth or prove a hindrance to plantingor maintenance to operations. The planting soil must be free of plant or seed material of non-nativeinvasive species or noxious weeds. Sediment run-on from disturbed areas can clog practices and changethe soils composition and pH. Also, compaction due to construction disturbance can also impact theireffectiveness and result in the necessity of spending more resources to restore their functionality.

This chapter will describe management techniques required to achieve the practice/management goal.The amount of required techniques depends on the initial condition and the management goal of eachproject (see Figure 5.1). As example, it takes more steps to get to good forest condition from compactedsoil than from good turf.

Management is a clearly defined state of soil and vegetation that provides the desired degree of infiltrationand pollutant removal under the optimal design condition. The design condition is assumed to be 15 yearsfollowing stabilization, when the site has reached a reasonable level of maturity and is undergoing onlygradual changes. Management is the defined desired endpoint and depends on a series of techniques toget from the current possibly highly disturbed condition to that endpoint. A management has clearlydefined specifications including soil hydrologic characteristics and vegetation type and density. The termcover is sometimes used to describe a management that has minimal inputs.

A Technique as pertaining to permanent stormwater management is a method or operation that progressesor sustains progress from one state of management to a higher-functioning state of management. Thesecan fall in two general categories:

1) Methods of getting from “here” to “there,” from the presumed worst-case condition immediatelyfollowing development to the desired management endpoint. The techniques used in a specificsite design will vary greatly depending on the starting conditions. For example, if an area of “goodforest” is left undisturbed, no technique is necessary to achieve a “good forest” management.On the other hand, if the post-construction condition is a bare highly-disturbed mixture of topsoiland subsoil that is heavily compacted, the required technique to achieve a “good forest” managementin 15 years may include all or some of the following: soil ripping, soil amendments, temporaryvegetative cover, slope erosion control, planting of trees, fertilization and irrigation.

2) The operations necessary to maintain the required trajectory towards the desired design condition,and to maintain that condition once it is achieved. In other words, the protocol for maintenanceof a “good forest” management may include the techniques of tree thinning, fertilization, andinvasive species removal. Note that the protocol for a “fair forest” management may include thesame techniques, but with less intensive requirements.

Timing of SCM installation and protection of the SCM area are two critical techniques. Installing SCMsprior to adequate site preparation may result in failure due to sedimentation and/or soil compaction.Timing of their installation is important in protecting these SCMs and avoiding sometimes extensive post-construction rehabilitation or complete re-installation. Infiltration SCMs depend on the quality of soilsand their characteristics of porosity, organic matter, pH, and texture. SCM performance is highly sensitiveto compaction or deviation from good growing conditions. For example, a bioretention facility shouldhave a mixture of sand, loam and compost. No other materials or substance should be mixed into thesemedia because it may be harmful to plant growth or prove a hindrance to maintenance and operation.The planting soil should be free of plant or seed material of non-native invasive species or noxious weeds.

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Sediment run-on from disturbed areas can clog practices and change soil composition or pH. Compactiondue to construction disturbance can also impact their effectiveness and result in the necessity of spendingmore resources to restore their functionality.

Management may be thought of as a spectrum, and techniques are used to mitigate impacts expressedin a site’s initial condition to achieve a goal of a managed vegetated area or other end management. Initialconditions are described in the left side of Figure 5.1, and a list of techniques used to move from the impactedend of the spectrum to the optimal soil condition are also listed. On the right side is the spectrum ofmanaged vegetated, and below it is a list of techniques used to establish the desired vegetation.

Figure 5.1: Spectrum of initial condition managements and permanent managements along withassociated techniques used to improve from a least optimal state to a more optimal state.

Techniques are used to achieve a management goal. The amount and rigor of technique(s) applied dependson the initial condition and the management goal of each project element. For example, it takes theapplication of more techniques to establish a good forest condition from compacted bare soil than froma porous clay soil with established temporary cover. Soil ripping or removal of compaction is needed in thecompacted clay scenario before fertilization techniques could be applied and a forest stand established.

Soil quality and vegetation are key factors in the effectiveness of an SCM at reducing runoff volume andremoving pollutants. Soils must possess the structure and texture that allows water to infiltrate and plantroots to penetrate. Design of stormwater management systems is conducted through modeling thatassumes disturbed soils are ameliorated back to a condition that resembles an undisturbed natural soil.A designer is responsible for documenting the initial condition and including in their plan the necessarytechnique(s) for soil amelioration and plant establishment to accomplish the projected target managementfor each element containing disturbed soils. This amelioration process should be included in the projectapplication as part of the permanent stormwater management plan.

5.3.1 Erosion Prevention and Sediment ControlsTo ensure the greatest potential of success of a LID facility, SCMs should not be installed until constructionactivity is completed and stabilized within the entire contributing drainage area; this means having all areasboth fully landscaped and mulched or having well established grass or other ground cover. Construction

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drawings should clearly state the designer's intentions and an appropriate stage of construction shouldbe shown on the plans. This staging should be covered in detail at the pre-construction meeting (includingthe on-site responsible construction personnel) and then enforced by an appropriate inspection programthroughout the construction period. On-site education of contractor and/or subcontractors would alsobe advised. Storing and reestablishing top soil on site is important to reestablishing the overall infiltrationof the site. Each site is unique, and this strict construction staging approach may not be necessary, suchas in the case of bioretention in parking lot islands with little contributing pervious areas. It is critical thatthe designer understands these realities and plans accordingly. For more details, refer to TDEC E&SHandbook chapter 6 and 7 (TDEC E&S Handbook p:94).

5.3.2 Soil RestorationSoil restoration is a technique used to enhance andrestore soils by physical treatment and/or mixturewith additives – such as compost – in areas wheresoil has been compacted. Soil media restorationincreases the water retention capacity of soil, reduceserosion, improves soil structure, immobilizes anddegrades pollutants (depending on soil mediamakeup), supplies nutrients to plants, and providesorganic matter. Soil restoration is also used toreestablish the soil’s long term capacity for infiltrationand to enhance the vitality of the soil as it hosts allmanner of microbes and plant root systems incomplex, symbiotic relationships. .

Restored soils result in increased infiltration anddecreased volume of runoff. Designers can receivecredit based on areas (acres) complying with the requirements of desired SCMs. For example, a bare soilcan be assigned a management reflecting a “good” condition instead of “poor” condition.

A healthy soil (Figure 5.3) provides a number of vital functions including water storage and nutrientstorage, regulate the flow of water, and immobilize and degrade pollutants. Healthy soil contains a diversecommunity of beneficial microorganisms, a sufficient amount of plant nutrients (nitrogen andphosphorous), some trace elements (e.g., calcium and magnesium), and organic matter (generally fiveto 10%).

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Figure 5.3: A healthy soil profile(Source: USDA NRCS ).

Figure 5.4: Compacted soil constraints movement of air and water (Source: USDA NRCS).

Figure 5.2: Soil amended with compost (Source: USDA NRCS).

Healthy soil typically has a neutral or slightly acidic pH and good structure which includes various sizes ofpores to support water movement, oxygenation, and a variety of other soil processes.

Caring for soil is also a critical component of water management, especially during development activities,such as construction grading, which often result in erosion, sedimentation, and soil compaction. Properprotection and restoration of soil are critical management techniques to combat these issues. Soil restorationprevents and controls erosion by enhancing the soil surface to prevent the initial detachment andtransport of soil particles.

Soil compaction is the enemy of water quality protection. Soil compaction occurs when soil particles arepressed together, reducing the pore space necessary to allow for the movement of air and water throughoutthe soil (Figure 5.4). This decrease in porosity causes an increase in bulk density (weight of solids per unitvolume of soil). The greater the bulk density of the soil means the lower the infiltration and the largerthe volume of runoff (Table 5.3).

Compaction limits vegetative root growth, restricting the health of plants as well as the biological diversityof the soil. Compaction also affects the infiltrating and water quality capacity of soils. Soil compactioncan lead to increased erosion and stormwater runoff, low infiltration rates, increased flooding, and decreasedwater quality from polluted runoff. After compaction, a typical soil has strength of about 6,000 kilopascals(kPa), while studies have shown that root growth is not possible beyond 3,000 kPa. . There are two typesof compaction, minor and major, each of which requires a particular restoration technique(s) or method:

• Minor compaction – Surface compaction within 8-12 inches due to contact pressure and axleload <20 tons can compact through root zone up to one foot deep. Soil restoration activitiescan include: subsoiling, organic matter amendment, and native landscaping. Tilling/scarifyingis an option as long as it is deep enough (i.e., 8-12 inches) and the right equipment is used(should not be performed with common tillage tools such as a disk or chisel plow because theyare too shallow and can compact the soil just beneath the tillage depth).

• Major compaction – Deep compaction, contact pressure and axle load > 20 tons can compactup to two-feet deep (usually large areas are compacted to increase strength for paving andfoundation with overlap to “lawn” areas). Soil restoration activities can include: deep tillage,organic matter amendment, and native landscaping.

To evaluate the level of compaction in soils, bulk density field tests are conducted. Table 5.3 shows theideal bulk densities for various textures of soils.

Table 5.3: Bulk density for soil texture (Source: USDA NRCS).

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Soil Texture Ideal Bulkdensities, g/cm3

Bulk densities thatmay affect rootgrowth, g/cm3

Bulk densitiesthat restrict rootgrowth, g/cm3

Sands, loamy sands <1.60 1.69 1.8

Sandy loams, loams <1.40 1.63 1.8

Sandy clay loams, loams, clay loams <1.40 1.6 1.75

Silt, silt loams <1.30 1.6 1.75

Silt loams, silty clay loams <1.10 1.55 1.65

Sandy clays, silty clays, some clay loams(35-45% clay) <1.10 1.49 1.58

Clays (>45% clay) <1.10 1.39 1.47

5.3.2.1 ApplicationsSoil restoration can occur anywhere to alleviate soil compaction. It can be specifically addressed in thefollowing examples:

• new development (residential, commercial, industrial) – Heavily compacted soils can be restoredprior to lawn establishment and/or landscaping to increase the porosity of the soils and aid inplant establishment.

• detention basin retrofits – The inside face of detention basins is usually heavily compacted,and tilling the soil mantle will encourage infiltration to take place and aid in establishing vegetativecover.

• golf courses – Using compost as part of landscaping upkeep on the greens has been shown toalleviate soil compaction, erosion, and turf disease problems.

5.3.2.2 Removal of compaction Table 5.4 describes various soil disturbance activities related to land development, soil types and therequirements for soil restoration for each activity. Soil Restoration or runoff reduction is a requiredpractice. Restoration is applied across areas of a development site where soils have been compacted andwill be vegetated according to the criteria defined in Table 3. Compacted soil can be amended by firsttilling the soil, breaking apart the compaction, and then applying various soil media.

Table 5.4: Soil Restoration Recommendations [NY, 2010. P:99].

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Type of SoilDisturbance Soil Restoration Requirement Comments/Examples

No soildisturbance Restoration not permitted Preservation of Natural Features

Minimal soildisturbance Restoration not required Clearing and grubbing

Minor soilcompaction

Six inches of soil media (18.5 cubic yardsper 1,000 square feet of soil) should beapplied, and then tilled into the existingsoil up to eight inches

Protect area from any ongoingconstruction activities.

Major soilcompaction

10 inches of soil media (31 cubic yards per1,000 square feet of soil) should be appliedand then tilled into the existing soil up to20 inches

Areas whereRunoff Reductionand/or Infiltrationpractices areapplied

Restoration not required, but may be appliedto enhance the reduction specified forappropriate practices.

Keep construction equipment fromcrossing these areas. To protect newlyinstalled practice from any ongoingconstruction activities construct a singlephase operation fence area

Redevelopmentprojects

Soil Restoration is required on redevelopmentprojects in areas where existing imperviousarea will be converted to pervious area.

• Tilling consideration:

Tilling the soil (also referred to as aeration, scarification, ripping, or subsoiling):a. Effective when performed on dry soils.b. Should be performed where subsoil has become compacted by equipment operation, dried out,

and crusted, or where necessary to obliterate erosion rills.c. Should be performed using a solid-shank ripper and to a depth of 20 inches, (eight inches for

minor compaction).d. Should be performed before amending media is applied and after any excavation is completed.e. Should not be performed within the drip line of any existing trees, over underground utility

installations within 30 inches of the surface, where trenching/drainage lines are installed, wherecompaction is by design, and on inaccessible slopes.

f. The final pass should be parallel to slope contours to reduce runoff and erosion.g. Tilled areas should be loosened to less than 1,400 kPa (200 psi) to a depth of 20 inches below

final topsoil grade.h. The subsoil should be in a loose, friable condition to a depth of 20 inches below final topsoil

grade and there should be no erosion rills or washouts in the subsoil surface exceeding threeinches in depth.

i. Tilling should form a two-directional grid. Channels should be created by a commerciallyavailable, multi-shanked, parallelogram implement (solid-shank ripper), capable of exerting apenetration force necessary for the site.

j. No disc cultivators, chisel plows, or spring-loaded equipment should be used for tilling. The gridchannels should be spaced a minimum of 12 inches to a maximum of 36 inches apart, dependingon equipment, site conditions, and the soil management plan.

k. The channel depth should be a minimum of 20 inches or as specified in the soil managementplan. If soils are saturated, delay operations until the soil, except for clay, will not hold a ballwhen squeezed.

l. Only one pass should be performed on erodible slopes greater than one vertical to threehorizontal.

5.3.2.3 Soil preparation for planting5.3.2.3.2 Lime, fertilizer, and topsoil applicationWhen conventional seeding is to be used, topsoil should be applied to any area where the disturbanceresults in subsoil at the final grade surface. Figure 3 provides guidance on the volume of topsoil requiredto provide specific topsoil depths. Soil pH should be above 5 – preferably between 6.0 and 6.5. Soil shouldbe submitted to a soils specialist or County Agricultural Extension agent for testing and soil amendmentrecommendations. In the absence of soil test results, the following application rates can be used:

• Ground agricultural limestone:Light-textured, sandy soils: 1- 1 1/2 tons/acre.Heavy-textured, clayey soils: 2-3 tons/acre.

• Fertilizer:Grasses: 800-1200 lb/acre of 10-10-10 (or the equivalent).Grass-legume mixtures: 800-1200 lb/acre of 5-10-10 (or the equivalent) (TDEC E&S Handbookp:119).

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• Topsoil

Table 5.5: Cubic Yards of Topsoil Required to Attain Various Soil Depths (TDEC E&S Handbook, P:120).

Depth (in) Per 1,000 Square feet Per acre

1 3.1 134

2 6.2 268

3 9.3 403

4 12.4 537

5 15.5 672

6 18.6 806

Organic Media Inorganic Media

Compost* Vermiculite

Aged manure* Perlite

Biosolids* (must be a Grade 1 biosolid) Pea gravel

Sawdust (can tie up nitrogen and cause deficiency in plants) Sand

Wood ash (can be high in pH or salt)

Wood chips (can tie up nitrogen and cause deficiency in plants)

Grass clippings

Straw

Sphagnum peat (low pH)

5.3.2.3.3 Herbicide applicationApplication of herbicides: This is a method of last resort, but necessary in some cases. Herbicidetreatments should be applied only to a specific plant and never broadcast, especially near water bodies.Use a colored dye in the herbicide mix to identify areas that have been sprayed. Use the least persistentpesticide available to accomplish the job.

5.3.2.4 Soil amendmentSoil media used for amendment may be comprised of either organic or inorganic material (table 3).Organic media can increase soil organic matter content, which improves soil aeration, water infiltration,water and nutrient holding capacity, and is an important energy source for bacteria, fungi, and earthworms.

Table 5.6: Restoration Media (MI, 2011).

* Materials containing animal wastes can cause phosphorus

• Soil amendment considerationsApplying soil media for amendment:a. Soil media should not be used on slopes greater than 30 percent. In these areas, deep-rooted

vegetation can be used to increase stability.b. Soil restoration should not take place within the critical root zone of a tree to avoid damaging

the root system. To determine the critical root zone, measure the tree diameter four and

half feet above grade. For every inch of diameter measured, the critical root zone will be onefoot radius from the tree. (For example, a tree that measures 20 inches in diameter will havea critical root zone of 20 foot radius).

c. Onsite soils with an organic content of at least five percent can be stockpiled and reused toamend compacted soils, saving costs. Note: These soils must be properly stockpiled tomaintain organic content.

d. Soils should generally be amended at about a 2:1 ratio of native soil to media. If a proprietaryproduct is used, follow the manufacturer’s instructions for the mixing and application rate.

e. Add six inches compost or other media and till up to eight inches for minor compaction. (Sixinches of compost equates to 18.5 cubic yards per 1,000 square feet of soil.)

f. Add 10 inches compost or other amendment and till up to 20 inches for major compaction.10 inches of compost equates to approx. 30.9 cubic yard per 1,000 square feet.

g. Compost can be amended with bulking agents, such as aged crumb rubber from used tires,or wood chips. This can be a cost-effective alternative that reuses waste materials whileincreasing permeability of the soil.

h. Compost shall be aged, from plan or dust produced when handling, pass through a half inchscreen and have a pH suitable to grow desired plants.

5.3.2.5 Mulch and other cover applications (TDEC E&S Handbook, p:104)Surface mulch is considered the most effective, practical means of controlling runoff and erosion ondisturbed land prior to vegetative establishment. Mulch reduces soil moisture loss by evaporation,prevents crusting and sealing of the soil surface, moderates soil temperatures, provides a suitablemicroclimate for seed germination, and may increase the infiltration rate of soil.

Straw mulch is the most common type of mulch used in conjunction with seeding or providing atemporary groundcover. The straw should come from wheat or oats (“small grains”), and may be spreadby hand or with a mulch blower. Note that straw may be lost to wind and must be tacked down. Therecommended application rate for straw mulch is 2 tons per acre, dry unchopped, unweathered.

Mulch, as well as wood chip, bark chips, shredded bark, wood fiber, and other material are temporarystabilization practices. Those materials can only increase initial condition of unprotected bare soil toprotected bare soil.

5.3.2.6 Winter considerationsSince soil restoration is performed in conjunction with plantings, this management technique should beundertaken in spring or autumn and during dry weather, so that plantings can establish.

5.3.2.7 Other considerations • During periods of relatively low to moderate subsoil moisture, the disturbed subsoils are

returned to rough grade and the following soil restoration steps applied:1. Apply 3 inches of compost over subsoil2. Till compost into subsoil to a depth of at least 20 inches using a cat-mounted ripper, tractor

mounted disc, or tiller, mixing, and circulating air and compost into subsoils3. Rock-pick until uplifted stone/rock materials of four inches and larger size are cleaned off the

site4. Apply topsoil to a depth of 6 inches5. Vegetate as required by approved plan.

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Figure 5.5: Attachments used for soil decompaction.

At the end of the project an inspector should be able to push a 3/8” metal bar 12 inches into the soil justwith body weight. Figure 5.5 shows two attachments used for soil decompaction (NY, 2010, p:100).

• Soil restoration may need to be repeated over time, due to compaction by use and/or settling.Taking soil core samples will help to determine the degree of soil compaction and if additionalmedia application is necessary. Two points help ensure lasting results of decompaction:1. Planting the appropriate ground cover with deep roots to maintain the soil structure2. Keeping the site free of vehicular and foot traffic or other weight loads. Consider pedestrian

footpaths. (Sometimes it may be necessary to de-thatch the turf every few years) [MI, P:298].

5.3.3 Native Vegetation RestorationNative species are generally described as those existing in a given geographic area prior to Europeansettlement. Over time, native vegetation does not typically require significant chemical maintenance byfertilizers and pesticides. This results in additional water quality benefits. Native species are typicallymore tolerant and resistant to pest, drought, and other local conditions than non-native species. Inaddition to chemical applications, minimum maintenance also means minimal mowing and irrigation inestablished areas. Native grasses and other herbaceous materials that do not require mowing or intensivemaintenance are preferred.

“Restoration” implies returning a landscape to a former, more pristine state. In reality, historic conditionscannot be replicated. For most development and redevelopment projects, a realistic goal is to remove ormitigate destructive impacts and reintroduce significant missing processes and components, wherepossible. The intent of these actions is to allow natural processes to bring about gradual recovery.

While there are many benefits to improving existing native cover types, the primary purpose of thismanagement technique is to increase the potential for effective stormwater management on a site andto provide the developer with another means of stormwater management. This management techniquefunctions by reestablishing a healthy plant community with thick, spongy soil layers that:

• Generates less runoff. • Absorbs a greater volume of water through infiltration, evaporation, and evapotranspiration. • Improves soil conditions through the addition of organic material, which increases soil pore

space. • Reduces the need for maintenance by fertilizers, herbicides, and pesticides. • Reduces the force of precipitation by leaf interception.

5.3.3.1 VariationsSpecies selection for any native landscape should be based on function, availability, and level of appropriatenessfor site conditions. Native species plantings can achieve variation in landscape across a variety ofcharacteristics, such as texture, color, and habitat potential.

Properly selected mixes of flowering prairie species can provide seasonal color; native grasses offerseasonal variation in texture. Seed production is a food source for wildlife and reinforces habitat. In allcases, selection of native species should strive to achieve species variety and balance, avoiding creationof single-species or limited species “monocultures” which pose multiple problems. In sum, many differentaspects of native species planting reinforce the value of native landscape restoration, typically increasingin their functional value as species grow and mature over time. Examples include:

• Prairie – Install Big Bluestem, Little Bluestem, Indian Grass, Switchgrass and others that resemblethe Native Americans grassland [Shea, 1999]. Prairies have a tendency to establish and regainfunction rather quickly (3-10 years), and can provide lower-growing vegetation with highlyattractive native grasses and wildflowers.

• No-mow lawn area – Install low-growing native grasses that are used as a substitute for lawnor cool-season grass plantings.

• Woodland – Install a balance of native trees, shrubs, forbs, grasses, and sedges. Woodlands willprovide shade, vertical structure, and a high level of rainfall interception in the long term. Ittypically requires a significant amount of time to mature.

• Constructed wetlands – Historic drained wetlands or existing artificial low areas may be plantedwith wetland species that will thrive in standing water or saturated conditions.

• Buffer areas – Bands of re-established native vegetation occurring between impermeablesurfaces, lawns, or other non-native land uses and existing natural areas.

• Replacement lawn areas – Existing turf lawns may be converted to native prairies, wetlands, orwoodlands to minimize maintenance while increasing stormwater benefits and wildlife habitat.

Figure 5.6: Tennessee native no-mow lawn and woodland (Source: The SMART Center).

5.3.3.2 CalculationNative revegetation and reforestation will increase infiltration capacity and reduce runoff volume.Designers can receive managed vegetated area credit based on the square feet of trees or shrubs beingadded. The credit is reflected through label “Good, fair, or poor”, that is described below:

– Good condition• ≤ 200 ft2 / tree, or expected full canopy cover• ≤ 25 ft2 / shrub• > 90% turf cover, with no continuous bare areas• Trees or shrubs with lush undergrowth, with > 90% of surface under either canopy or ground cover

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– Fair condition• 200 - 350 ft2 / tree, or expected canopy cover > 75%• 25 - 40 ft2 / shrub• > 75% turf cover, with no contiguous bare area > 50 ft2

• Trees or shrubs with fair undergrowth, with > 75% of surface under either canopy or groundcover

– Poor condition• 350 - 500 ft2 / tree, or expected canopy cover > 50%• 40 - 60 ft2 / shrub• > 50% turf cover, with no contiguous bare area > 75 ft2

• Trees or shrubs with some undergrowth, with > 50% of surface under either canopy or groundcover

– Below these targets, is not considered an adequate measure for which to claim credit, due toboth very limited infiltration capacity and potential to serve as a source of TSS

– Minimum seeding/planting will receive “poor” credit, while optimize seeding/planting willreceive “fair” credit. “Good” credit can only be achieved when optimized seeding/planting isfollowed by a maintenance practice (see Figure 5.1).

5.3.3.3 MaterialsWhenever practical, native species should be from the same ecoregion as the project area. Whennecessary, species may be used from adjacent ecoregions for aesthetic or practical purposes. Informationrelating to Tennessee native species and their use in landscaping is available from on Appendix D.

Developments should use native trees for replacement in areas separate from residential lots, or stormdrainage areas adjacent to roadway or parking lots. Species selection shall be based on the underlyingsoils and the historic, native indigenous plant community type for the site, if existing conditions cansupport the plant community.

Native plan restoration/reforestation is eligible under the following qualifying conditions:• Avoid the use of a single species of tree. No more than 20% of the area composes of any single

tree species. Reforestation should consider the composition of area forests, and two thirds ofselected trees must be large canopy. Reforestation methods should achieve full canopy coverwithin ten to fifteen years.

• The minimum size requirement for reforestation is saplings 6-8 feet in height. The minimumsize requirement for shrubs is 18-24 inches, or 3 gallon size. In addition, the entire reforestationshould be covered with 2-4 inches of organic mulch or with a native seed mix in order to helpretain moisture and provide a beneficial environment for the reforestation.

• The trees must be free from injury, pests, diseases, and nutritional disorders; and must be fullybranched and have a healthy root

• A long-term vegetation management plan must be prepared and included in the site’smaintenance agreement in order to demonstrate the ability to maintain the reforestation areain an appropriate forest canopy condition. The plan should include a scale drawing showing thearea to be planted, along with a plant list which includes species, size, number, and packaging.In addition, the reforestation area shall be clearly identified on all construction drawings andEPSC plans during construction.

• The reforestation area must be protected in perpetuity such that no future development ordisturbance may occur within the area.

• The planting plan must be approved by the local jurisdiction including any special sitepreparation needs.

• It is recommended that the construction contract contain a care and replacement warrantyextending at least two growing seasons, to ensure adequate growth and survival of the plantcommunity.

• The final size of the trees should be considered when designing the planting plan. TennesseeOne-Call (811) must be contacted prior to the submission of the planting plan to ensure thatno utilities will be impacted by the tree planting. The planting plan must also avoid placing treesunder overhead utilities.

5.3.3.4 Restoration process1. Assess vegetation onsite and delineate areas to be preserved. Note landscape cover type, size,

condition, and age. 2. Integrate areas selected for protection into site and stormwater plans to meet multiple

objectives and create environmental and social connections (trail systems, hedgerows, etc.). 3. Identify glaring problems within the selected enhancement area, e.g., fill and soil pushed over

the slope into the valley, extreme cut, exposed subsoil or bedrock, bare and eroded soil, invasiveexotic plants, trash, and toxic materials. Particularly note erosion and sedimentation problemssuch as gullies and bare soil. Also note influences beyond the site that may undermineenhancement efforts.

4. Identify and address factors that suppress regeneration of native plants or contribute tooverall plant community decline before replanting or amending the soil. If these factors arenot addressed, efforts spent on enhancement will be wasted.

5. Where relevant, identify major cyclical processes that shape the site, e.g., floods, fire, etc.These recurring natural events may help to sustain the native plant community and preventcolonization by invasive exotics.

6. Search for a healthy model in the neighborhood to serve as a design reference:– Plant community structure and pattern—Use the model to determine the arrangement, types,

and density of plants. – Identify and protect desirable and sensitive species and any rare, threatened, or endangered

plant (or animal) species. Particularly identify “keystone” species. If absent, replace thesespecies where possible.

– An unusual amount of dead or dying plants requires a determination of cause. 7. General Recommendations

– In healthier systems with minimal disturbance, native seeds may be present in the soil. Areasadjacent to other healthy natural areas can benefit from seeds transported by wind, water,and animals. If time is not a factor, and rapid cover is not critical, these areas can be left toregenerate on their own.

– Plant tough, vigorous, generalist species, which will create immediate cover and discourageinvasive species.

– Stabilize edges. Where a remnant natural area meets a manmade landscape, the designshould create a graceful, smooth transition. Construction often leaves these transition areashighly disturbed. Repair of these newly exposed edges is critical.

– Regrade where necessary, stabilize the soil, and replant with fast-growing, tough, native edgespecies. Repair of damaged edges will protect the health of the natural landscape andenhance its stormwater benefits.

– Newly exposed, existing trees are often vulnerable to wind throw. Replant a strip along newlyformed edges (where a portion of the natural landscape has been cut away) to buffer theremaining native landscape from increased wind, light, noise, and other impacts.

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5.3.3.5 Tree planting guidelines

Figure 5.7: Tree planting guidelines.

For more guidance about planting tree, visit:

http://www.tn.gov/agriculture/publications/forestry/treeline_hbook.pdf

http://www.arborday.org/trees/planting/index.cfm

http://www.tn.gov/twra/pdfs/treeplanting.pdf

http://www.oregon.gov/odf/privateforests/docs/ReforestationGuide.pdf

5.3.4 Maintenance Practices The requirements for the Maintenance Document are in Appendix F .They include the execution andrecording of an Inspection and Maintenance Agreement or a Declaration of Restrictions and Covenants,and the development of a Long Term Maintenance Plan (LTMP) by the design engineer. The LTMP containsa description of the stormwater system components and information on the required inspection andmaintenance activities. The property owner must submit annual inspection and maintenance reports tothe local stormwater program.

First year maintenance operations includes– Initial inspections for the first six months (once after each storm greater than half- inch).− Reseeding to repair bare or eroding areas to assure grass stabilization.− Water once every three days for first month, and then provide a half in year. Irrigation plan may

be adjusted according to the rain event.− Fertilization may be needed in the fall after the first growing season.

Ongoing maintenanceTwo points help ensure lasting results of decompaction:

1. Planting the appropriate ground cover with deep roots to maintain the soil structure2. Keeping the site free of vehicular and foot traffic or other weight loads. Consider pedestrian

footpaths. (Sometimes it may be necessary to de-thatch the turf every few years)− Soil restoration may need to be repeated over time, due to compaction by use and/or settling.

Taking soil core samples will help to determine the degree of soil compaction and if additionalmedia application is necessary.

– Mowing is permitted but not encouraged between the trees while they are being established.Eventually, the canopy should shade out the grass and forest undergrowth will be establishedremoving the need to mow. Vegetation management plans should consider if residents wouldprefer the site mowed in perpetuity.

– Additional maintenance activities include:• Watering the trees as needed.• Repairing areas of erosion or reseeding areas that are bare.• Removing trash and debris from area.• Replanting any trees that die throughout the year (the construction contract should contain

a care and replacement warranty extending at least two growing seasons, to ensure adequategrowth and survival of the plant community).

• Addressing areas of standing water which might breed mosquitoes.• Picking up branches that have fallen.• Grooming trees or shrubs as needed.• Removing any trees or limbs damaged in storms that might pose a danger.

Removing invasive species• Where necessary, remove masses of aggressive, invasive exotic species to expose the potential

of the area. − Invasive exotic species often occur as dense shrub thickets or extensive, heaping vine cover.

Vines in trees and climbing over shrubs suppress reproduction on the ground and shade oldertrees and shrubs, eliminating seed sources. Privet, Japanese honeysuckle, kudzu, mimosatrees, and tree of heaven are the most prevalent invasive exotics.

− Remove large tangles of aggressive exotic species to allow an accurate evaluation of the siteand suggest appropriate repair strategies.

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General considerations: − Effective control treatments vary by species. In some cases, non-chemical options exist.

o Emphasize techniques that minimize soil disturbance and that remove the exotic plants bythe roots where possible, while leaving adjacent, desirable plants undamaged. Whenremoving existing invasive plants, either pull up by the roots or eliminate re-sprouts later.

− Some invasive exotics are more troublesome than others. For example, highly aggressivespecies such as kudzu and privet are particularly difficult to eradicate and should be removedas early as possible, before they are well established.

− Phase removal of exotic canopy trees to keep a shady forest cover. Specific removal methods

− Hand pulling: Suggested for restricted areas of herbaceous weeds or small seedlings of woody plants. − Tools: A weed wrench allows the user to pull up young tree seedlings (too large to pull by hand)

by the roots. This tool disturbs the adjacent areas only minimally. − Mowing: In general, broadleaved herbaceous plants will diminish with regular mowing. Broadleaved

herbaceous plants include both weeds and wildflowers, and a meadow mown more than threetimes a year will become predominantly grasses.

− Controlled burning: This technique can be used to manage any landscape cover type. However,in meadow management, fire is used to reduce the number of trees and increase the amountof grasses and wildflowers. o Burning can be used in two major ways: 1) If the remnant natural area is small, or only a small

portion of it requires treatment, a single person, with a backpack propane torch, can burn smallareas (approximately 10 feet by 10 feet). Generally, small-scale burns are done as a patchworkof squares, with unburned vegetation between burned patches. Extreme caution must beused to prevent wildfire; 2) Where a relatively large-scale burn is considered (approximately1 acre or more), property managers should coordinate with the local fire department andstate conservation agencies. Permits are required from the local government.

o Caution: Some undesirable species, such as black locust, are “fire increasers.” If these speciesare already present, burning may encourage them. Conversely, successful regeneration ofoak forests in the eastern United States has historically required fire.

− Tilling: For large areas of infestation, tilling can uproot and kill undesirable species. However,tilling can also kill native species and encourage invasive plants that spread by undergroundrhizomes or stolons, such as kudzu (Pueraria lobata).

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R E F E R E N C ES

Center for Watershed Protection. 2010. New York State Stormwater Management Design Manual.Tech. Albany, NY: Department of Environmental Conservation. P:97-102.

Chattanooga Hamilton County Regional Planning Commission (CHCRPC). 2012. Rainwater ManagementGuide. Chattanooga, TN.

King, D., P. Hagan, 2011. Costs of Stormwater Management in Maryland Counties. University ofMaryland, Solomons, MD.

Metropolitan Government of Nashville and Davidson County. 2012. Low Impact Development:Stormwater Management Manual. Nashville, TN: Metro Water Services. P:213.

North Carolina Division of Water Quality. 2012. “Chapter 4: Selecting the Right BMP.” NCDENRStormwater BMP Manual.

Prince George’s County, Maryland, Department of Environmental Resources. 2000. Low ImpactDevelopment Design Strategies: An Integrated Design Approach. Prince George’s County, Maryland,Department of Environmental Resources, Programs and Planning Division, Largo, MD.

SEMCOG. 2008. A Design Guide for Implementers and Reviewers Low Impact Development Manualfor Michigan. Rep. Detroit, MI. P:122-123; 293-288.

Tennessee Department of Environment and Conservation (TDEC). 2012. Tennessee Erosion & SedimentControl Handbook: A Stormwater Planning and Design Manual for Construction Activities. FourthEdition. August 2012.

USDA Natural Resources Conservation Service (NRCS). A Soil Profile. 5 Dec. 2014.

USDA NRCS. St. Croix Watershed Restoration Project: Hope & Carton Hill Rain Garden. 5 Dec. 2014.

Documents

Permanent Stormwater Management Measures